An inclusion is a solid particle in liquid aluminium alloy. It is usually non-metallic and can be of different nature depending on its source. Problems related to inclusions Inclusions can create problems in the casting when they are large and in too high concentration. Here are examples of problems related to inclusions: Pinholes in light gauge foil Flange cracks in beverage containers Surface streaks in bright automotive trim and lithographic material Breakage in wire drawing operation Increased tool wear and tear Increased porosity Loss of pressure tightness of engine blocks Poor machinability Cosmetic defect in apparent surfaces Diminished mechanical properties (e.g. Ultimate Tensile Strength, Yield Strength, Elongation) Inclusion types Oxide films In contact with ambient air, liquid aluminium reacts with the oxygen and form an oxide film layer (gamma-Al2O3). This layer becomes thicker with time. When molten aluminium is disturbed, this oxide film gets mixed inside the melt. Aluminium carbide In primary aluminium production, aluminium carbides (Al4C3) originates from the reduction of alumina where carbon anodes and cathodes are in contact with the mix. Later in the process, any carbon tools in contact with the liquid aluminium can react and create carbides. Magnesium oxides In aluminium alloys containing magnesium, magnesium oxides (MgO), cuboids (MgAl2O4-cuboid) and metallurgical spinel (MgAl2O4-spinel) can form. They result from the reaction between magnesium and oxygen in the melt. More of them will form with time and temperature. Spinel can be highly detrimental because of its big size and high hardness. Refractory materials Particles of refractory material in contact with aluminium can detach and become inclusions. We can find graphite inclusions (C), alumina inclusions (alpha-Al2O3), CaO, SiO2, … After some time, graphite refractory in contact with aluminium will react to create aluminum carbides (harder and more detrimental inclusions). In aluminium alloy containing magnesium, the magnesium reacts with some refractories to create rather big and hard inclusions similar to spinels. Unreacted refractory particles can originate from the degradation of refractory materials which comes in contact with the melt. Chlorides Chloride inclusions (MgCl2, NaCl, CaCl2, …) are a special type of inclusion as they are liquid in liquid metal. When aluminium solidifies, they form spherical voids similar to hydrogen gas porosity but the void contains a chloride crystal formed when aluminium became colder. Fluxing salt Fluxing salt, like chlorides are also liquid inclusions. They come from flux treatments added to the melt for cleaning. Intentionally added inclusions Titanium boride